A function switching circuit in a conventional integrated circuit will be described in reference to FIG. 5. In FIG. 5, reference numeral 11 designates an OR circuit, reference numeral 12 designates an input interface circuit, reference numeral IN1 designates a signal input to the OR circuit 11 in the integrated circuit, reference numeral OUT1 designates an output from the OR circuit 11 and reference numeral IN designates a signal input to the OR circuit 11 of the integrated circuit. In addition, reference numeral N11 designates an output signal from the input interface circuit 12.
Operation thereof will be described hereinafter.
The signal IN input becomes the signal N11 through the input interface circuit 12 and it is input to the OR circuit 11. When the signal IN is "High", the output OUT1 of the OR circuit 11 becomes "High" regardless of the input signal IN1. Alternatively, when the signal IN is "Low", the input signal IN1 appears in the output OUT1 of the OR circuit 11. Thus, switching between whether the signal IN1 is output or "High" is always output as the output signal OUT1 from the OR circuit 11 is performed in accordance with the input signal IN.
FIG. 6 is a view showing an example when the OR circuit 11 is constructed using a junction type field effect transistor (referred to as a JFET hereinafter). In FIG. 6, the same reference numbers as in FIG. 5 represent the same parts or parts having the same function. Reference numerals J11 to J16 designate JFETs, reference numerals R11 and R12 designate load resistors, reference numerals IN1 and IN1B designate the signal IN1 and its inverted signal in the circuit shown in FIG. 5 and reference character VDD designates a power supply voltage.
Next, operation thereof will be described hereinafter. The JFETs J13 and J16 are constant current sources and the load resistors R11 and R12 and the JFETs J11 and J12 switch in a differential manner and the JFETs J15 and J16 constitute a source follower circuit for outputting the switched signal.
First, when the signal N11 is "Low", lower than the signals IN1 and IN1B, the JFET J14 cuts off current, so that operation of the circuit is the same as when the JFET J14 is omitted. More specifically, the operation is the same as that of the normal source coupled type logic circuit. Its operation is as follows. That is, when a current flowing through one of the JFETs J11 and J12 whose gate voltage is higher with gate voltages IN1 and the IN1B, respectively, is increased, a large voltage drop occurs at the load resistor R11 or R12, and the resulting voltage is output through the source follower. For example, when the voltage of the IN1 is higher than that of the IN1B, a larger current flows through the JFET J11 and the current flowing through the JFET J12 is reduced, so that the voltage drop at the resistor R12 increases and the output voltage OUT becomes "High". Alternatively, when the voltage of the signal IN1 is lower than that of the signal IN1B, the current flowing through the J12 increases, so that the voltage drop at the resistor R12 is reduced and the output signal OUT becomes "Low".
When the N11 is "High", higher than IN1 and IN1B, since the J14 is always in a conductive state, a larger current flows through the resistor R11 and the current flowing through the resistor R12 is reduced regardless of the signals IN1 and IN1B and the output signal OUT becomes "High".
In order to operate as described above, as the signal N11, a voltage of "High", which is higher than the signal IN1 and the signal IN1B, and a voltage of "Low", which is lower than the signal IN1 and the signal IN1B are required, meaning that a large signal amplitude of approximately 2 V or more, is required. In addition, in a circuit in which its function can be switched from outside, a signal is input thereto from another outside circuit. When a CMOS IC is used as this circuit and when a 5 V supply voltage is used, the output from the CMOS IC is a voltage between 0 V and 5 V and a signal having a signal amplitude of 5 V from 0 V to 5 V is input as the signal IN.
Meanwhile, when the signal IN is directly input to the gate of the JFET, a gate current flows when the voltage between the gate and the source is higher than the gate junction voltage, which is approximately 1.2 V when the JFET is formed of GaAs, causing a large input current requirement. The circuit for switching the function could malfunction because of this current. In order to prevent this malfunction, the input interface circuit is provided as shown in FIG. 5.
FIG. 7 is a diagram showing a conventional input interface circuit. In FIG. 7, the same references as in FIG. 5 designate the same parts or parts having the same functions. In addition, reference numerals J21 and J22 designate JFETs and reference numeral D21 designates a diode.
Next, operation thereof will be described.
The voltage of the signal IN which is input from the outside causes a voltage drop in the voltage between the gate and the source (Vgs) of the JFET J21 at the JFET J21 and then a voltage drop in the forward direction voltage (Vf) of the diode D21 occurs at the diode D21, whereby the output signal N11 is obtained. As described above, even if the voltage of the output signal N11 is high, since the voltage is obtained from the following equation, that is, Vdd-Vgs-Vf, it will not be increased to the 5 V of the power supply voltage VDD. As a result, the above problems can be prevented.
However, since the conventional function switching circuit and the conventional input interface circuit for a function switching signal are constructed as described above, the current for the input interface circuit is required, whereby power consumption of the integrated circuit is increased.